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The presence of a specific binding protein for lead (7439921) in human the erythrocytes and the variations among individuals to synthesize this protein in response to a lead challenge was investigated as a result of observed discrepancies between total blood lead and clinical and biochemical indices of lead toxicity. Two studies were conducted using groups of exposed and unexposed lead workers ranging from 25 to 45 years old. Blood lead measurements in the first study were obtained through lyophilized fractions by atomic absorption spectrophotometry with a granite furnace attachment and through atomic absorption spectrophotometry of hemoglobin A2 separated by cellulose chromatography. In the second study, membrane fractions of the erythrocytes obtained by centrifugation were measured by atomic absorption for lead content and by standard techniques for Na-K-ATPase activity. The presence of a 10,000 molecular weight lead binding protein in erythrocytes was found in lead workers, but not in normal subjects. The researcher concludes that there is a subgroup of industrial workers particularly susceptible to lead intoxication because of their inability to bind lead to this protein. Further study is suggested to investigate the possibility that the circulating erythrocyte is the source of the 10,000 molecular weight binding protein, and whether there is a genetic basis for the failure to synthesize the protein in response to a lead challenge.